Soil Moisture Constants: The water contents expressed under certain standard conditions are commonly
referred to as soil moisture constants. They are used as reference points for practical irrigation water
management. The usage of these constants together with the energy status of soil water gives useful
knowledge.
Different Soil Moisture
1. Saturation capacity
2. Field capacity
3. Permanent wilting point
4. Available soil moisture
5. Hygroscopic coefficient
1. Saturation capacity
Saturation capacity refers to the condition of soil at which all the macro and micro pores are filled with water and the soil is at maximum water retention capacity" The matric suction at this condition is essentially zero as the water is in equilibrium with free water. Excess water above saturation capacity of soil is lost from root zone as gravitational water.
2. Field capacity
According to Veilmeyer and Hendrickson (1950) the field capacity is "the amount of water held in soil after excess water has been drained away and the rate of downward movement has materially decreased which usually takes place within 1-3 days after a rain or irrigation is pervious soils having uniform texture and structure (Fig. 9.1). At field capacity, the soil moisture tension depending on the soil texture ranges from 0.10 to 0.33 bars for-10 to -33 kPa). Field capacity is considered as the upper limit of available soil moisture. The field capacity is greatly influenced by the size of the soil particles (soil texture), finer the soil particles higher the water retention due to very large surface area and vice versa. Thus, at field capacity, a m3 of a typical sandy soil will hold about 135 liters of water, a loamy soil about 270 liners and a clay soil about 400 liters.
3. Permanent wilting point
It is the condition of the soil wherein water is held so tightly by the soil particles that the plant roots can no longer obtain enough water at a sufficiently mid rate to satisfy the transpiration needs to prevent the leaves from wilting. When this condition is reached the soil is said to be in a state of permanent willing point, at which nearly all the plants growing on such soil show wilting symptoms and do not revive in a dirk humid chamber unless water is supplied from an external source (Fig 9.1). The soil moisture tension at permanent wilting point is about 15 hoes (or-1500 kPa) equal to a suction or negative pressure of a water column 1.584 x 104 cm (pF = 42). Permanent wilting point is considered as lower limit of available soil moisture. Under field conditions PWP is determined by growing indicator plant as sunflower in small containers. In the laboratory pressure membrane apparatus can be used to determine the moisture content at 15 bars
4. Available soil moisture
It has been a convention and even now it is a customary to consider "the amount of soil moisture held between the two cardinal points viz, field capacity (0.33 bars) and permanent wilting point (15 bars) as available soil moisture"
Though considerable soil moisture is present below the permanent wilting point, it is held to tightly by the soil particles that the plant roots are unable to extract it rapidly enough to prevent wilting. Then, practically it is not useful for the plants and forms the lower limit of available oil moisture. Similarly, the water above the field capacity is not available to the plants owing to quick drainage. The available soil moisture is expressed as depth of water per unit of soil and is calculated according the following formula:
Available Soil Moisture (mm/depth of soil)
= (FC-PWP) X pb X ds
10
Where,
FC = Field capacity moisture (%) on oven dry weight basis
PWP = Permanent wilting point moisture (%)on oven dry weight basis
pb = soil bulk density (g/cm³)
ds = Depth of soil (cm)
ASM = Available soil moisture (mm/m depth of soil)
5. Hygroscopic coefficient
It is defined as the amount of water that the soil contains when it is in equilibrium with air at standard atmosphere i.e., 98% relative humidity and at room temperature. In other words it is the amount of moisture absorbed by a dry soil when placed in contact with an atmosphere saturated with water vapour (100% relative humidity) at any given temperature, expressed in terms of percentage on an oven dry basis. The matric suction of soil water at this moisture content is nearly about 31 bars.
2) Enlist different Criteria for Scheduling Irrigation. Explain various approaches of Scheduling Irrigation.
Different Criteria for Scheduling Irrigation.
A) Soil moisture regime approaches
1. Soil water content approach
2. Depletion of available soil moisture
3. Soil water potential or soil moisture tension
B) Climatological approaches
1. PET measurement
2. IW/CPE approach
C) Plant indicator approaches
1. Visual plant symptoms
2. Plant water content
3. Leaf diffusion resistance
4. Plant temperature
5. Critical crop growth stage
Explanation
1) Soil moisture depletion approach:
The available soil moisture in the root is a good criterion for scheduling irrigation. When the soil moisture in a specified root zone depth is depended to a particular level (which is different for different crops) it is too replenished by irrigation. For practical purpose, irrigation should be started when about 50 percent of the available moisture in the soil root zone is depleted. The available water is the soil moisture, which lies between field capacity and wilting point. The relative availability of soil moisture is not same field capacity to wilting point stage and since the crop suffers before the soil moisture reaches wilting point, it is necessary to locate the optimum point within the available range of soil moisture, when irrigation must be scheduled to maintain crop yield at high level. Soil moisture deficit represents the difference in the moisture content at field capacity and that before irrigation. This is measured by taking into consideration the percentage, availability, tension, resistance etc.
3) Define Water use efficiency. Explain measures to improve water use efficiency.
Water Use Efficiency (WUE)
It is the yield of marketable crop produced per unit water used in evapo-transpiration (ET). It is expressed in kg/ha-mm.
Water use efficiency of are two types as
(i) crop water use efficiency
(ii) field water efficiency.
A. Climatic factors:
Plant transpiration and soil evaporation are dependent upon the temperature, wind velocity, relative humidity, sunshine hours and rainfall of a particular area. Evapotranspiration is directly correlated with temperature and wind velocity thereby reducing WUE. Similarly, evaporation is inversely proportional to humidity of climate which results in reduced consumption of water thereby increasing water use efficiency. Increased availability of light to plants increases photosynthesis resulting in greater production which consequently increases WUE of crops.
B. Nature of crops:
Crops with higher canopies have greater growth and consequently higher photosynthesis which results in greater yield and concomitant higher WUE. Plants with shallow and less developed roots are able to absorb less water and fertilizers resulting in their lesser growth and production. Consequently, their WUE is reduced.
C. Cultural practices:
1) Sowing time:
The crops sown at proper time have greater production and hence higher water use efficiency. The crops grown latter have lesser growth and development produce low yield and hence lesser WUE.2) Method of sowing:
Compared to broadcasting method of sowing of crops, line sowing of crops has greater utilization and absorption of nutrients, water and light resulting in higher production which results in higher WUE. Grain yield of wheat, oats and pearlmillet were also increased when crops were sown in the N-S direction.
3) Depth of sowing:
Crops whose seeds are sown at optimum depth have greater growth since germination and hence higher production resulting in greater WUE.
4) Use of antitranspirants:
Antitraspitants are those materials whose spray upon plants reduced transpiration. Kaolin, phenyl mercuric acetate and abscisic acid are a few well known anti-transpirants. The spraying of anti-transpirantsupon plants results in their reduced transpiration which lessons their consumptive use thereby increasing WUE.
5) Use of growth retardants:
Experiments have proved that there exist certain chemical substances like cycocel (CCC), phosphon etc. whose praying upon plants in good production despite lack of water. Hence, it generates higher WUE.
6) Use of mulch:
Mulches refers to the artificial or natural materials covered on the surface of soil with a object to reduce evaporation and destruction of weeds resulting in greater use of light, fertilizers, air and water by crops which results in higher production consequently higher WUE.
4) Describe the role of Water in Plants.
A) Physiological importance
- The plant system itself contains about 90% of water
- It acts as base material for all metabolic activities.
- It plays an important role in respiration and transpiration
- It plays an important role in photosynthesis It plays in important role in plant metabolism for vegetative and reproductive growth
- It serves as a solvent in soil for plant nutrients
- It also acts as a carrier of plant nutrients from soil to plant system
- It maintains plant temperature through transpiration
- It helps to keep the plant erect by maintaining plant's turgidity
- It helps to transport metabolites from source to sink
B) Ecological Importance
- It helps to maintain soil temperature
- It helps to maintain salt balance
- It reduces salinity and alkalinity
- It influences weed growth
- It influences atmospheric weather
- It helps the beneficial microbes
- It influences the pest and diseases It supports human and animal life
- It helps for land preparation like ploughing, puddling, etc.,
- It helps to increase the efficiency of cultural operations like weeding, fertilizer application etc., by providing optimum condition.
5) Write Classification of Soil Water with its characteristics.
A) Physical classification: Based on relative degree of retention,
1. Gravitational water
2. Capillary water
3. Hygroscopic water
1. Gravitational water (Drainage water)
It is usually present in macro pores.
It held by a negative tension of less than 0.3 atm.
Free water which drains out.
It is not available to plants and detrimental to plants
It dissolves plant nutrients & these nutrients leached away.
2. Capillary water
It is present in micropores of soil
Held between field capacity and hygroscopic co-efficient in micropores.
At tension of 0.3 to 31 atm.
Function as soil solution
It is available for plant growth.
Capillary water divided in to inner capillary and outer capillary water:
Inner capillary water: It is that part of capillary water which is nearer to the hygroscopic water. It is in the form of thinner films, held more tightly and moves rather very slow than outer capillary water.
Outer capillary water: It is that part of capillary water which is not very tightly held in the soil and therefore moves readily from place to place. It is the most useful water for the plants because of its very quick availability.
3. Hygroscopic water
This water is held very tightly as thin film around soil particles by adsorption forces and therefore plant can not absorb it.
It held at hygroscopic co-efficient.
Tension varies from 31 to 10,000 atm.
Moves in vapour form therefore biologically inactive.
B) Biological classification of soil water: (Basis of available to plants)
There is a certain relationship between soil moisture retention and its utilization by plants. On the basis, superfluous, available water and unavailable water are recognized.
1. Superflous water (Free water or drainable water)
It is excess of that held in soil at field capacity and has no use for higher plants. This water is held at a tension below 0.1 to 0.3 atmosphere. It is also known as free or gravitational water.
2. Available water (Available soil moisture, ASM)
Soil moisture between field capacity and permanent wilting point is referred to as readily available moisture. It is the moisture available for plant use. Held between 0.3 to 15 atm.
3. Unavailable water
It is held in the soil at the permanent wilting point (≥15 atm.) unavailable water includes hygroscopic water and that part of the capillary water held between 15 to 31 atmosphere which is utilized by plants too slowly to prevent wilting.
Unavailable water (cm/depth)
= PWP (%)
------------- x BD x D
100
6) Enlist methods of Irrigation system. State advantages and disadvantages of Drip & Sprinkler irrigation.
Methods of irrigation
1. Surface methods-
a. Wild flooding
b. Check basin
c. Ring Basin method
d. Border strip
e. Furrow method-1. Corrugation
f. Surge irrigation
2. Sub surface irrigation
3. Sprinkler/ overhead irrigation
4. Drip/ Trickle irrigation
5. Automated Irrigation System
Advantages of Drip Irrigation
1. Well suited for areas of acute water shortage.
2. Minimization of soil erosion and deep percolation and runoff losses.
3. Water is maintained at field capacity.
4. Salt concentration is less.
5. No land leveling is necessary.
6. Herbigation and Fertigation can also be applied.
7. Less disease and weed infestation.
Disadvantages of Drip Irrigation
1. High initial cost on plastic pipes as extensive pipe net work is needed.
2. Drippers are susceptible to blockage and difficult to locate the clogging.
3. Interferes with farm operations and movement of implements and machineries.
4. Frequent maintenance is required.
5. Requires the clean water for irrigation.
6. Not suitable for closed spaced crops.
7) State causes of Water logging and write management of water logged soils.
Advantages of Sprinkler System
1. Uniform distribution of water.
2. Saving of water from 25-50 per cent.
3. Saving of land 10-20 per cent.
4. Irrigation area is increased by 1-2 times with the same amount of the water.
5. No risk of runoff and erosion.
6. Suitable for undulating land and steep sloppy.
7. Suitable for areas where water and labour scarcity.
8. Suitable for saline soils to leach salts.
Disadvantages of Sprinkler System
1. Not followed under high wing velocity (>12 km/hour).
2. High initial costs.
3. High energy is required (0.50 to >10 kg/cm2).
4. More spreading of diseases.
5. Can not be used for rice and jute crops.
7) State causes of Water logging and write management of water logged soils.
Causes of water logging
1. Excessive use of water when the water is available in abundance or cheaply due to the belief that more water contributes better yield.
2. Improper selection of irrigation methods.
3. Percolation and seepage from lands canals and reservoir located at nearby elevated places.
4. Improper lay out and lack of outlets.
5. Presence of impervious layer with profile impeding percolation.
6. Upward rise of water from shallow ground water table or aquifer.
7. Ingress of sea water to adjacent lands.
8. Floods in rivers spread to cultivated land during high storms and cyclones causes water logging.
Drainage System:
Soil Texture and Structure Improvement:
Raised Beds:
Selecting Suitable Plants:
Planting Time and Density:
Contour Plowing:
Subsurface Irrigation:
Avoiding Over-Irrigation:
Terracing:
Mulching:
Tiling:
Proper Land Grading:
Monitoring and Observation:
Avoiding Soil Compaction:
Avoiding Water-Intensive Crops:
8) Write the process of Water absorption by plants. State factors responsible for water absorption.
Absorption of Water:
In plants, water is absorbed through root hairs, which are in contact with soil water. The wall of the root hairs are permeable and consists of pectic and cellulose substances which are strongly hydrophilic (water loving) in nature. There are two types of absorption viz., (a) Active absorption, and (b) Passive absorption.
(a) Active absorption – Active transport depending on expenditure of metabolic energy mass flow move same direction in mass. Here the process of osmosis plays an important role. The soil plant water movement can be effected due to forces of imbibition, diffusion and osmosis.
(b) Passive absorption – Passive movement by mass flow and diffusion without spending energy. Passive absorption takes place when rate of transpiration is very high. It is otherwise known as transpiration pull.
Factors responsible for water absorption
Water absorption by plants is influenced by environmental and plant factors. The former includes mainly the atmospheric and soil factors.
(i) Available soil water - Capillary water is available to plants. Hygroscopic water and gravitational water are not available to plants. The capillary water is absorbed by the plants, which in turn reduces the soil water potential. Hence, the water from higher potential area tends to move to lower potential area and root will absorb this water. This is the chain of process involved in water uptake.
(ii) Concentration of soil solutions - High concentration affects the process of osmosis.
(iii) Soil air - Sufficient amount of O2 should be there and excess amount of CO2 affects the availability of water by root suffocation.
(iv) Soil temperature - Up to 30oC favours absorption. Very low and very high temperature affects absorption.
(v) Soil texture
Clay – Neither good nor bad
Sand – Not good for absorption
Loamy – Good for absorption
9) Enlist different methods of Soil Moisture estimation and explain in short the pressure plate apparatus methods.
Methods of Soil Moisture Estimation:
A) Direct method:
(1) Gravimetric Method (Oven dry method):
(2) Volumetric Method
B) Indirect methods:
1) Tensiometer (Irrometers)
2) Gypsum block/Resistance block
3) Pressure membrane and pressure plate apparatus
4) Neutron Moisture Meter (neutron scattering method)
5) Microwave remote sensing
6) Time-domain Reflactometer (TDR)
10) Water requirement of crops. Describe the factors affecting water requirement.
Crop water requirement (WR) implies the total amount of water required at the field head regardless of its source, to mature a crop. It includes the amount of water needed to meet the losses through evapo-transpiration, application lossess and the special needs like leaching of excess salts, puddling, pre-planting irrigation etc. The water requirement (WR) thus can be expressed as: WR = CU + Application losses + Water needed for special operation WR= Irrigation + Effective rainfall + Ground water contribution + Change in soil moisture
Factors influence the crop water requirements are:
1) Crop factors
a) Crops and varieties: tall crops and varieties intercept more solar radiation and have more ET than short crops and varieties.
b) Growth stages
c) Duration: Longer duration, higher the water requirement.
d) Plant population
e) Crop growing season
f) Rooting habits: Deep rooted crops extract more water from deep soil layers.
2) Soil factors
a) Soil Structure
b) Soil Texture
c) Soil Depth
d) Soil Topography
e) Soil chemical composition
f) Hydralulic conductivity: Coarse textured soils have higher hydraulic conductivity than fine textured soil at high soil moisture regime.
g) Reflectivity:
h) Thermal properties
3) Climatic factors
a) Temperature
b) Sunshine hours/solar radiation
c) Relative humidity
d) Wind velocity
e) Rainfall Higher solar radiation, temperature and wind velocity increase crop water needs. Higher humidity reduces ET. Hot and dry winds around irrigated crop increases the ET.
4) Agronomic management factors
a) Irrigation methods used
b) Frequency of irrigation and its efficiency
c) Tillage and other cultural operations like weeding, mulching etc / intercropping etc
